The invention relates to the printing industry and more specifically to loading of unexposed printing plates onto an exposure device. Printing plates are usually shipped in cardboard containers with paper sheets, also known as slip sheets, separating the plates. The paper sheets sometimes adhere to the front or back of the plate, in particular when plates are sheared to size with the paper sheets in place. In a manual operation the operator has no difficulty seeing when a paper sheet adheres to the plate and has to be removed. In an automated system the reliable handling of the plates in the presence of slip sheets is a problem, as the automated system has difficulty detecting if a slip sheet is fully removed. The slip sheets come in many colors and textures, some hard to tell apart from the plate. As the plate manufactures can change the type and color of paper used as slip sheets at any time reliable detection based on color and/or surface texture is not possible. The consequences of an undetected slip sheet are serious, as the slip sheet can be passed on to the plate processor where it contaminates the processor.
After successful removal of slip sheet and verification that no paper remained attached to either front or back of plate, the plate is loaded onto the imaging unit for exposure. As the image has to be registered to the edge of the plate, the method used for edge detection is critical. When the imaging device is a drum type, reliable clamping means are required to accommodate many plate sizes on a single exposure device. Prior art plate handling equipment suffers from three main limitations:
A. Lack of reliable means to detect the absence of paper from both sides of the plates. PA0 B. Lack of ability of handling a continuously range of plate sizes, due to need for fixed clamps. PA0 C. The need to use special cassettes instead of loading directly from shipping boxes.
It is the object of this invention to overcome these limitations and reliably load printing plates onto an imaging device, thus increasing the degree of pre-press automation. A further object is to provide a flexible system capable of handling a continuously variable range of plate sizes and thicknesses.